Stress Migration (SM) in Copper is a reliability failure mechanism where copper atoms diffuse due to mechanical stress gradients — typically tensile stress that develops during cooling from processing temperatures, causing void nucleation and growth near via connections.

What Is Stress Migration?

• Cause: CTE mismatch between Cu ($alpha approx 17$ ppm/°C) and dielectric ($alpha approx 0.5$ ppm/°C). Cu wants to contract more than the dielectric allows -> tensile stress in Cu.
• Voiding: Atoms migrate toward free surfaces (via bottoms, grain boundaries) to relieve stress, leaving voids behind.
• Temperature: Worst case at intermediate temperatures (~150-250°C) where diffusion is active but stress is not fully relaxed.

Why It Matters

• Wide Lines: Counterintuitively, SM is worse in wider metal lines (more total stress, more atoms available to migrate).
• Burn-In: Can be triggered or accelerated by burn-in testing conditions.
• Design Fix: Redundant vias and via-array rules reduce SM risk.

Stress Migration is thermal contraction pulling copper apart — a mechanical stress-driven failure where the mismatch between copper and glass tears the metal from within.